Constant current line circuit with class features

ABSTRACT

A line circuit utilizing a differential amplifier whose output is regulated to zero volts D.C. (when the subscriber&#39;s set is off-hook) by a negative feedback circuit, for controlling a voltage controlled power supply which provides a constant current feed to the subscriber&#39;s loop. The differential amplifier also transmits audio signals received from the subscriber&#39;s loop directly to a PABX via an unbalanced terminal. When the loop range exceeds a predetermined threshold, the circuit reverts to a voltage feed operation thereby allowing operation over long loop lengths and the ability to provide on-hook CLASS services.

FIELD OF THE INVENTION

The present invention relates to telephone line circuits, and moreparticularly to a constant current line circuit with on-hooktransmission for CLASS service delivery.

BACKGROUND OF THE INVENTION

Modern day PABXs supply operating power to subscribers' lines at −48volts, applied to tip and ring leads of the lines. The length of asubscriber's line or subscriber loop governs its resistance. Hence, thecurrent drawn by a long subscriber loop is substantially less than thatdrawn by a short loop.

Prior art line circuits typically did not compensate for different looplengths and usually provided a constant voltage battery feed to the tipand ring leads. A minimum loop current of approximately 20 mA istypically required to be provided to a subscriber's loop in order tooperate a telephone set connected to the loop. For long loops, largevoltages were required in order to generate the minimum 20 mA current.Conversely, considerable power was wasted for short subscriber loops asa result of driving the line with unnecessarily high voltages (eg. −48volts, −96 volts, etc.).

The problem of prior art line circuit power regulation is exemplified bythe well known transformer hybrid which includes a large magnetic corefor passing D.C. feed current to the loop. The core is made large inorder not to saturate in the presence of large D.C. feed currents. Priorart transformer hybrids were bulky and expensive, and thus did notconform to miniaturization requirements of modern day PABXs.

Commonly assigned U.S. Pat. No. 4,723,280 (Meier) sets forth a constantcurrent line circuit which utilizes a differential amplifier fortransmitting audio signals to and from a subscriber's loop as well asdetecting the amount of feed current flowing in the loop. Thedifferential amplifier compares the detected feed current to a thresholdvalue for establishing the constant current, and generates a D.C.voltage signal which varies in amplitude in response to variations inthe amount of feed current flowing in the loop, resulting fromvariations in loop resistance, etc. The D.C. signal is passed through anintegrator circuit in order to remove audio signal components and isapplied to a voltage controlled D.C. power supply for increasing ordecreasing feed voltage applied to the loop in response to a decrease orincrease respectively in the feed current flowing in the loop, therebymaintaining constant feed current. The D.C. signal generated by thedifferential amplifier is regulated by the integrator circuit to aquiescent value of approximately zero volts, such that audio signalsreceived from the subscriber's loop can be applied directly to anunbalanced transmit lead connected to the PABX, without requiringseparate D.C. blocking signal translation circuitry.

The prior art constant current feed driver suffers from two significantweaknesses which prevent it from being used to provide CLASS services(e.g. caller ID). The first problem is that the ring driver is driven tosaturation in the on-hook state. This prevents the transmission of thecaller ID information. The second weakness is that the circuit ceases tofunction if the constant current requirement cannot be satisfied (e.g.if the loop range exceeds the design criteria or the terminal device ordevices require more current than the circuit can provide).Specifically, once the ring driver is driven to saturation, the zerovolt quiescent output of the differential amplifier cannot bemaintained, resulting in incorrect tip bias.

SUMMARY OF THE INVENTION

According to the present invention, a line circuit is provided whichoperates as a constant current loopfeed circuit in the short loopcondition. The constant current operation is as set forth in commonlyowned U.S. Pat. No. 4,723,280 (Meier), the contents of which areincorporated herein by reference. However, according to the presentinvention the circuit is prevented from entering the saturation state asthe looplength approaches and exceeds a predetermined amount, andinstead reverts to a voltage feed operation. This allows the circuit tooperate on longer loops as well as enabling on-hook transmission forCLASS service delivery.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of the preferred embodiment is set forth hereinbelow with reference to the sole drawing, in which:

FIG. 1 is a schematic diagram of a constant current line circuit withCLASS features, according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, Tip and Ring terminals are provided forconnection to tip and ring leads of a subscriber's loop connected to aremote telephone set. The Tip and Ring terminals are shown connected tofeed resistors 1 and 2. Inverting and non-inverting inputs of adifferential amplifier 3 are connected to the Ring and Tip terminals viainput resistors 4 and 5, respectively.

The inverting and non-inverting inputs of differential amplifier 3 arealso connected in respective circuit paths to feed resistors 1 and 2 viacurrent sense resistors 7 and 8, respectively. The inverting input ofamplifier 3 is connected to a source of reference voltage Viloop, viaresistor 9.

The output of differential amplifier 3 is connected to a transmit audiosignal terminal VX for passing audio signals received from thesubscriber's loop to a PABX.

The output of differential amplifier 3 is also connected to an invertinginput of a differential amplifier 10 via a resistor 11. Thenon-inverting input of differential amplifier 10 is connected to ground,and the output is connected to the inverting input via a capacitor 12.Differential amplifier 10 is thus configured as an integrator circuit,in a well known manner.

The output of differential amplifier 10 is a also connected via resistor13, to an inverting input of a differential amplifier 14. Anon-inverting input of differential amplifier 14 is connected to asource of bias voltage to bias the operating voltage of the ring driverby creating an offset voltage relative the ring driver voltage when theintegrator saturates, as discussed in greater detail below. An output ofdifferential amplifier 14 is fed back to the inverting input viafeedback resistor 20. The resistances of resistors 13 and 20 establishthe gain of amplifier 14. The output of amplifier 14 is also connectedto the base of PNP transistor 15, an emitter terminal of which isconnected to the ring feed junction of feed resistor 2 and current senseresistor 8. A collector terminal of transistor 15 is connected to the−48 V battery voltage. Thus, amplifier 14 and transistor 15 operate inconjunction as a ring driver for providing battery voltage to the ringlead of the subscriber's loop.

A receive audio signal on terminal VX is also applied to thenon-inverting input of amplifier 24 via capacitor 29A and resistor 29B.Capacitor 29A prevents the voltage VX from altering the tip bias. Aninverting input of differential amplifier 24 is connected to ground viaresistor 25. An output of differential amplifier 24 is fed back to theinverting input thereof via resistor 27. The resistances of resistors 25and 27 establish the gate of amplifier 24. The output of amplifier 24 isalso connected to a base terminal of an NPN transistor 28 and to theinverting input of ring drive amplifier 14. A collector terminal oftransistor 28 is connected to ground, and an emitter terminal thereof isconnected to the node connecting feed resistor 1 and current senseresistor 7 (designated as the tip feed point). Thus, amplifier 24 andtransistor 28 operate in conjunction as a tip driver. The tip driver isbiased externally (not shown in the drawings).

Considering operation of the circuit for providing constant feedcurrent, differential amplifier 3 senses the current flowing throughfeed resistor 2 by detecting the voltage thereacross via approximatelyequal valued input resistors 4 and 8, and the voltage across feedresistor 1 is detected via approximately equal valued input resistors 5and 7. The voltages across feed resistors 1 and 2 are summed indifferential amplifier 3 and compared therein with the Viloop referencevoltage generated by the PABX and applied to the inverting input ofamplifier 3 via resistor 9.

In the event of a sudden change in loop current, (e.g. due to thetelephone set in the subscriber's loop going off-hook), differentialamplifier 3 generates a D.C. voltage resulting from the comparisonbetween the sensed loop current and the desired loop current (asdetermined by the reference voltage applied to the Viloop terminal). TheD.C. voltage is applied to ring drive amplifier 14, via the integratorcircuit, for varying the voltage applied to the ring terminal via feedresistor 2 and thereby providing a constant current feed at the desiredcurrent.

Differential amplifier 10 functions as an integrator circuit forremoving A.C. audio signal components on the Tip and Ring leads andreceived by differential amplifier 3. Because the non-inverting input ofdifferential amplifier 10 is connected to ground, the D.C. voltage onthe inverting input thereof is drawn toward zero volts. Hence, theoutput voltage of differential amplifier 3 is regulated to a quiescentvalue of approximately zero volts.

The voltage output from differential amplifier 10 is received byamplifier 14, which in response drives the base terminal of PNPtransistor 15. As the voltage applied to the base terminal changes, thecollector current flowing in transistor 15 varies in response. Thecollector current flowing through transistor 15 drives the ring feedpoint.

According to the present invention, the line circuit enters a voltagefeed mode when the integrator 10 saturates in response to the ringdriver reaching its maximum positive output capabilities. The integrator10 operates in saturation mode as the loop range extends beyond constantcurrent design capabilities for the circuit. Thus, the integrator 10saturates as it's output approaches the positive supply voltage. Sincethe ring drive amplifier 14 does not saturate prior to the integrator 10saturating, the ring driver continues to operate in the linear regionand the circuit thereby reverts to a voltage feed operation. While theintegrator 10 is operating in saturation, it is no longer capable ofresponding to the error voltage presented by amplifier 3 for maintainingconstant current supply. As a result, the integrator 10 output voltageremains constant as it's saturation voltage. The ring driver 14maintains this fixed voltage level and the loop current becomes therebydependant on the voltage feed level and the loop resistance. Whenoperating in saturation mode, the circuit of the present inventiontreats an on-hook state as a very long loop and it is by this means thaton-hook audio transmission is enabled.

In summary, the line circuit of the present invention utilizes adifferential amplifier whose output is regulated in zero volts D.C.(when the subscriber's set is off-hook) by a negative feedback circuit,for controlling a voltage controlled power supply which provides aconstant current feed to the subscriber's loop. The differentialamplifier also transmits audio signals received from the subscriber'sloop directly to a PABX via an unbalanced terminal. When the loop rangeexceeds a predetermined threshold, the circuit reverts to a voltage feedoperation thereby allowing operation over long loop lengths and theability to provide on-hook CLASS services.

Alternative embodiments and variations of the invention are possible,all of which are believed to be within the sphere and scope of theclaims appended hereto.

1. A line circuit for providing loopfeed current to a subscriber's line,comprising: driver circuitry for driving said subscriber's line withsaid loopfeed current; an input amplifier circuit connected to saidsubscriber's line for sensing and comparing current flowing in said linewith a predetermined constant current and generating an error signalproportional to any difference therebetween; and an integrator forreceiving said error signal and in the event said error signal is lessthan a predetermined amount then controlling said driver circuitry witha variable voltage so as to maintain constant loopfeed current to saidsubscriber's line and in the event said error signal exceeds saidpredetermined amount generating a constant saturation voltage forcontrolling said driver circuit to provide a constant voltage feed tosaid subscriber's line.
 2. The line circuit of claim 1, wherein saiddriver circuitry further comprises driver amplifiers for drivingtransistors connected to said subscriber's line.
 3. The line circuit ofclaim 2, further comprising feed resistors connected to saidsubscriber's line across which voltage develops which is proportional tosaid current flowing in said line, said feed resistors being connectedin a circuit to differential inputs of said input amplifier circuit fordetection of said voltage.
 4. The line circuit of claim 3, furthercomprising input and current sense resistors connected across said feedresistors and to said differential inputs of said input amplifier. 5.The line circuit of claim 4, further comprising a source of referencevoltage proportional to said predetermined constant current applied toone of said differential inputs of said input amplifier.